Earthquake Protection Systems_2 docx

The magnitude of an earthquake is a measure of its total size, the energy released at its source as estimated from instrumental observations.. Average Number of Earthquakes Occurring Eac

Figure 1.2 The collapse of masonry buildings is the cause of most of the deaths in earthquakes around the world The 1982 Dhamar Earthquake, Yemen Arab Republic

continuing changes in the types of buildings being constructed in many of thecountries most at risk Modern building materials, commercialisation of the con-struction industry and modernisation in the outlook of town and village dwellersare bringing about rapid changes in building stock Brick and concrete block arecommon building materials in even the most remote areas of the world, and thewealthier members of rural communities who 20 or 30 years ago would havelived in weak masonry houses now live in reinforced concrete framed housesand apartment blocks

Unfortunately, many of the reinforced concrete framed houses and apartmentblocks built in the poorer countries are also highly vulnerable and, moreover,when they do collapse, they are considerably more lethal and kill a higher per-centage of their occupants than masonry buildings In the second half of thetwentieth century most of the urban disasters involved collapses of reinforced

concrete buildings and Figure 1.1 shows that the proportion of deaths due tocollapse of reinforced concrete buildings is significantly greater than earlier inthe century.

1.2.3 The World’s Earthquake Problem is Increasing

On average, about 200 large-magnitude earthquakes occur in a decade – about

20 each year Some 10% to 20% of these large-magnitude earthquakes occur inmid-ocean, a long way away from land and human settlements Those that occur

on land or close to the coast do not all cause damage: some happen deep in theearth’s crust so that the dissipated energy is dispersed harmlessly over a widearea before it reaches the surface Others occur in areas only sparsely inhabitedand well away from towns or human settlements

However, as the world’s population grows and areas previously with smallpopulations become increasingly densely settled, the propensity for earthquakes

to cause damage increases At the start of the century, less than one in three oflarge earthquakes on land killed someone The number has gradually increasedthroughout the century, roughly in line with the world’s population, until inthe twenty-first century, two earthquakes in every three now kill someone Theincreasing frequency of lethal earthquakes is shown in Figure 1.3

But the annual rate of earthquake fatalities does show some signs of beingreduced Figure 1.1 shows that the total number of fatalities in the years1950–1999 has averaged 14 000 a year – down from an average of 16 000 ayear in the previous 50 years And the number of earthquake-related fatalities in

Number of Fatal Earthquakes per Decade

steadily over the last century But the number per decade in which more than 1000 have been killed has remained roughly constant

the 1990s was 116 000, an average for the decade of 11 600 per year Some ofthis reduction is undoubtedly due to beneficial changes: the reduction in fatalitiesfrom fire is largely due to changes in the Japanese building stock and successfulmeasures taken by Japan to avoid conflagrations in its cities And changes inbuilding practices in some areas are making a significant proportion of buildingsstronger than they used to be.

Nevertheless the present worldwide rate of reduction in vulnerability appearsinsufficient to offset the inexorable increase in population at risk In the lastdecade the world’s populationwas increasing by about 1.5% annually, i.e dou-bling every 50 years or so, so the average vulnerability of the world’s buildingstock needs to be falling at a reciprocal rate, i.e halving every 50 years, simplyfor the average annual loss to be stabilised The evidence suggests that althoughthe average vulnerability of building stock is falling, it is not falling that quickly,

so that the global risk of future fatalities is rising overall

1.2.4 Urban Risk

Urban earthquake risk today derives from the combination of local city – the likelihood of a large-magnitude earthquake – combined with largenumbers of poorly built or highly vulnerable dwellings A detailed analysis of thelargest 800 cities in the world combining data on population, population growthrates, housing quality and global distribution of seismic hazard enables us toestimate the risks in all the large earthquake-prone cities, and compare them.Table 1.3 lists some of the world’s most highly vulnerable cities and dividesthem into risk categories Risk is here measured by the numbers of housing unitswhich could be destroyed in the event of the earthquake with a 10% probability

seismi-of exceedance in 50 years (approximately the once in 500 years earthquake).This assessment of loss is an indication of the overall risk, averaged out over along period of time The actual pattern of loss is likely to consist of long periods(a century or more) with small losses, with occasional catastrophic losses Of the

29 cities in the three highest risk categories, only 8 cities (6 in Japan and 2 inthe United States) are in the high-income group of countries; the 21 others areall in the middle- or low-income group of countries

It is clear from both Table 1.1 and Table 1.3 that the risk today is polarising,with industrialised countries obtaining increasing levels of safety standards intheir building stock while the increasing populations of developing countriesbecome more exposed to potential disasters This polarisation is worth examining

in a little more detail

1.2.5 Earthquake Vulnerability of Rich and Poor Countries

Earthquakes causing the highest numbers of fatalities tend to be those affectinghigh densities of the most vulnerable buildings In many cases, the most vulner-able building stock is made up of low-cost, low-strength buildings Some idea

Table 1.3 Cities at risk: the cities across the world with the highest numbers of dwellings likely to be destroyed in the ‘500-year’ earthquake.

(thousands)

Category A (over 25 000 dwellings destroyed in ‘500-year’ earthquake)

on information compiled by the authors from earthquake vulnerability surveys, recent earthquake loss experience and a variety of local sources of information The resulting estimates are very approximate.

of the cost and quality of building stock involved in these fatal events can beobtained by comparing the economic costs inflicted by the earthquakes (chieflythe cost of destroyed buildings and infrastructure) with human fatalities This ispresented in Figure 1.4, for the countries most affected by earthquakes in thetwentieth century.4

The highest casualties are generally those affecting low-cost construction InFigure 1.4, the economic losses incurred range from $1000 of damage for everylife lost (China) to over $1 million worth of damage for every life lost (USA).The location of individual countries on this chart is obviously a function of theirseismicity as well as the vulnerability to collapse of their building stock andthe degree of anti-seismic protection of their economic investment The mostearthquake-prone countries will be found towards the top right-hand corner ofthe chart, and the least towards the bottom left corner Richer countries will lieabove the diagonal joining these corners, poorer countries below it

In general, high-seismicity countries want to reduce both their total casualtiesand their economic losses In order to do this, those concerned with earthquake

San Salvador (3) Afghanistan (8) Algeria (6) Burma (4)

Colombia (15)

Taiwan (21) Mexico (19) Jamaica (2)

Albania (12)

Costa Rica (6) Nepal (2) India (9) Pakistan (8) Ecuador (17)

Indonesia (27) Philippines (20)

Turkey (68)

Iran (62)

China (64) Peru (31)

USSR (25) Guatemala (7) Rumania (2)

Nicaragua (4)

Chile (8) Italy (25) Japan (42)

Yugoslavia (10)

USA (40)

$1000

$10 000 Damage per Fatality: $ 1 million $ 100 000

Earthquake Losses by Country

Figure 1.4 Fatalities and economic loss in earthquakes by country (after Ohta et al.

1986)

4After Ohta et al (1986).

protection need first of all to understand some of the technical aspects of quake occurrence and the terminology associated with seismology, the study ofearthquakes There are a large number of books that explain earthquake mechanics

earth-in far greater detail than is possible here, and a number are listed earth-in the tions for further reading at the end of the chapter But some of the principles ofearthquake occurrence are worth summarising here, to explain the terminologywhich will appear in later chapters

1.3.1 Geographical Distribution of Earthquakes

The geographical distribution of earthquake activity in the earth’s crust is seenfrom the global seismic hazard map shown in Plate I The map shows the distribu-tion of expected seismicity across the earth’s surface, measured by the expectedintensity of shaking over a given time.5 The concentration of seismicactivity

in particular zones can be clearly seen Two features of this map are worthelaborating

1 Running down the western side of the Pacific Ocean from Alaska in the north

to New Zealand in the south is a series of seismic island arcs associated with

the Aleutian Islands, Japan, the Philippines and the islands of South East Asiaand the South Pacific; a similar island arc runs through the Caribbean andanother surrounds Greece

2 Two prominent earthquake belts are associated with active mountain building

at continental margins: the first is on the eastern shores of the Pacific stretchingthe length of the Americas, and the second is the trans-Asiatic zone runningeast–west from Myanmar through the Himalayas and the Caucasus Mountains

to the Mediterranean and the Alps

In addition to these major sources of earthquake activity, through the middle ofeach of the great oceans (but not shown on the map) there is a line of earthquakes,

which can be associated with underwater mountain ranges known as mid-ocean ridges Elsewhere, earthquakes do occur, but the pattern of activity is less dense,

and magnitudes are generally smaller

cohesive plates, forming the earth’s structure, floating on top of the mantle, the

hotter and more fluid layer beneath them Convection currents in the mantle causeadjoining plates to move in different directions, resulting in relative movementwhere the two plates meet This relative movement at the plate boundaries isthe cause of earthquakes The nature of the earthquake activity depends on thetype of relative movement At the mid-ocean ridges, the plates are moving apart.New molten rock swells up from below and forms new sea floor These areas

are called spreading zones At some plate boundaries, the plates are in head-on

collision with each other; this may create deep ocean trenches in which the rockmass of one plate is thrust below the rock mass of the adjacent plate The result ismountain building associated with volcanic activity and large earthquakes which

tend to occur at a considerable depth; these areas are called subduction zones.

The ocean trenches associated with the island arcs and the western shores ofSouth America are of this type Some collision zones occur in locations wheresubduction is not possible, resulting in the formation of huge mountain rangessuch as the Himalayas

There are also some zones in which plates are moving parallel and in oppositedirections to each other and the relative movement is primarily lateral Examples

of these are the boundary between the Pacific plate and the North American platerunning through California, and the southern boundary of the Eurasian plate inTurkey; in these areas large and relatively shallow earthquakes occur which can

be extremely destructive

Subduction Zones

The mid-ocean ridges are the source of about 10% of the world’s earthquakes,contributing only about 5% of the total seismic energy release By contrast, thetrenches contribute more than 90% of the energy in shallow earthquakes andmost of the energy for deeper earthquakes as well Most of the world’s largestearthquakes have occurred in subduction zones

Intra-plate Earthquakes

A small proportion of the energy release takes place in earthquakes located away

from the plate boundaries Most of such intra-plateearthquakes occur in

con-tinental zones not very far distant from the plate boundaries and may be theresult of localised forces or the reactivation of old fault systems They are moreinfrequent but not necessarily smaller than inter-plate earthquakes Some largeand highly destructive intra-plate earthquakes have occurred The locations ofintra-plate earthquakes are less easy to predict and consequently they present amore difficult challenge for earthquake protection

An important consequence of the theory of plate tectonics is that the rate anddirection of slip along any plate boundary should on average be constant over

a period of years In any given tectonic system, the total energy released in

earthquakes or other dissipations of energy is therefore predictable, which helps

to understand seismic activity and to plan protection measures Likely locations

of future earthquakes may sometimes be identified in areas where the energy

known to have been released is less than expected This seismic gap theory is a

useful means of long-term earthquake prediction which has proved valuable insome areas Earthquake prediction is discussed further in Chapter 3

1.3.2 Causes of Earthquakes

Earthquakes tend to be concentrated in particular zones on the earth’s surface,which coincide with the boundaries of the tectonic plates into which the earth’scrust is divided As the plates move relative to each other along the plate bound-aries, they tend not to slide smoothly but to become interlocked This interlockingcauses deformations to occur in the rocks on either side of the plate boundaries,with the result that stresses build up But the ability of the rocks to withstandthese stresses is limited by the strength of the rock material; when the stressesreach a certain level, the rock tends to fracture locally, and the two sides move

past each other, releasing a part of the built-up energy by elastic rebound

Once started, the fracture tends to propagate along a plane – the ruptureplane – until a region where the condition of the rocks is less critical has beenreached The size of the fault rupture will depend on the amount of stress build-upand the nature of the rocks and their faulting

1.3.3 Surface Faulting

In most smaller earthquakes the rupture plane does not reach the groundsurface, but in larger earthquakes occurring at shallow depth the rupturemay break through at the earth’s surface producing a crack or a ridge – a

surface break – perhaps many kilometres long A common misconception about

earthquakes is that they produce yawning cracks capable of swallowing people

or buildings At the epicentre of a very large earthquake rupturing the surface

on land – quite a rare event – cracks in the earth do occur and the ground eitherside of the fault can move a few centimetres, or in very large events a fewmetres, up or along This is, of course, very damaging for any structure that

is built straddling the rupture During the few seconds of the earthquake, theground is violently shaken and any fault rupture is likely to open up severalcentimetres in the shaking There is a slight possibility that a person could

be injured in the actual fault rupture, but by far the worst consequences ofdamage and injury come from the huge amounts of shaking energy released

by the earthquake affecting areas of hundreds of square kilometres This energyrelease may well cause landslides and ground cracking in areas of soft or unstableground anywhere in the affected area, which can be confused with surface faulttraces

1.3.4 Fault Mechanisms; Dip, Strike, Normal

According to the direction of the tectonic movements at the plate boundary thefault plane may be vertical or inclined to the vertical – this is measured by theangle of dip – and the direction of fault rupture may be largely horizontal, largelyvertical, or a combination of horizontal and vertical

The different types of source characteristic do produce recognisably differentshock-wave pulses, notably in the different directional components of the firstmoments of ground motion, but in terms of magnitude, intensity and spatialattenuation the different source mechanisms can be assumed fairly similar forearthquake protection planning

1.3.5 Earthquake Waves

As the rocks deform on either side of the plate boundary, they store energy – andmassive amounts of energy can be stored in the large volumes of rock involved.When the fault ruptures, the energy stored in the rocks is released in a fewseconds, partly as heat and partly as shock waves These waves are the earth-quake They radiate outwards from the rupture in all directions through the earth’s

crust and through the mantle below the crust as compression or body seismic

waves They are reflected and refracted through the various layers of the earth;when they reach the earth’s surface they set up ripples of lateral vibration orseismic waves which also propagate outwards along the surface with their own

characteristics These surface waves are generally more damaging to structures

than the body waves and other types of vibration caused by the earthquake Thebody waves travel faster and in a more direct route so most sites feel the bodywaves a short time before they feel the stronger surface waves By measuring thetime difference between the arrival of body and surface waves on a seismogram(the record of ground motion shaking some distance away) seismologists canestimate the distance to the epicentre of a recorded earthquake

1.3.6 Attenuation and Site Effects

As the waves travel away from the source, their amplitude becomes smaller andtheir characteristics change in other complex ways Sometimes these waves can

be amplified or reduced by the soils or rocks on or close to the surface at the site.Theground motion which we feel at any point is the combined result of the sourcecharacteristics of the earthquake, the nature of the rocks or other media throughwhich the earthquake waves are transmitted, and the interaction with the site effects

A full account of earthquake waves and their propagation is outside the scope

of this book, but is well covered elsewhere.6 The effect of site characteristics

on the nature and effects of earthquake ground motion is further discussed inChapter 7

6 See e.g Bolt (1999).

Not all earthquakes are tectonic earthquakes of the type described here Asmall but important proportion of all earthquakes occur away from plate bound-aries These include some very large earthquakes and are the main types ofearthquakes occurring in many of the medium- and low-seismicity parts of theworld The exact mechanisms giving rise to such intra-plate earthquakes are stillnot clearly established It is probable that they too are associated with faulting,though at depth; as far as their effects are concerned they are indistinguishablefrom tectonic earthquakes.

Earthquakes can also be associated with volcanic eruptions, the collapse ofunderground mine-workings, and human-made explosions Generally earthquakes

of each of these types will be of very much smaller size than tectonic quakes, and they may not be so significant from the point of view of earth-quake protection

earth-1.3.7 Earthquake Recurrence in Time

Given the nature of the large geological processes causing earthquakes, we canexpect that each earthquake zone will have a rate of earthquake occurrence asso-ciated with it Broadly, this is true, but as the rocks adjacent to plate boundariesare in a constant state of change, a very regular pattern of seismic activity is rarelyobserved In order to observe the pattern of earthquake recurrence in a particularzone, a long period of observation must be taken, longer in most cases than thetime over which instrumental records of earthquakes have been systematicallymade A statistical study of earthquake occurrence patterns, using both historicaldata and recent data from seismological instruments, can enable us to determineaverage return periods for earthquakes of different sizes (see Figure 1.5) This

is the approach which has been used to develop the global seismic hazard mapshown in Plate I and is discussed further in Chapter 7

1.3.8 Severity and Measurement of Earthquakes

The size of an earthquake is clearly related to the amount of elastic energyreleased in the process of fault rupture But only indirect methods of measuringthis energy release are available, by means of seismic instruments or the effects

of the earthquake on people and their environment

The terms magnitude and intensity tend to be confused by non-specialists in

discussing the severity of earthquakes The magnitude of an earthquake is a

measure of its total size, the energy released at its source as estimated from

instrumental observations The intensity of an earthquake is a measure of the

severity of the shaking of the ground at a particular location ‘Magnitude’ is aterm applied to the earthquake as a whole whereas ‘intensity’ is a term applied to

a site affected by an earthquake, and any earthquake causes a range of intensities

at different sites

Average Number of Earthquakes Occurring Each Decade

of Magnitude greater than or equal to M

Figure 1.5 Average recurrence rate of earthquakes of different magnitudes worldwide (after B˚ath 1979)

1.3.9 Earthquake Magnitude

A number of magnitude scales are in use The oldest is the Richter magnitude

(Ml) scale, defined by Charles Richter in 1936 It is based on the logarithm ofthe amplitude of the largest swing recorded by a standard seismograph Becauseearthquakes of different types cause different forms of seismic wave trains, more

detailed measurements include body wave magnitude ( mb ) and surface wave nitude ( Ms), based on the amplitudes of different parts of the observed wavetrain In general, the definition of magnitude which best correlates with the sur-face effects of earthquakes is the surface wave magnitude Ms, since it is thesurface waves which are most destructive to buildings There are a number ofcorrelations between the different magnitude definitions

mag-Because magnitude scales are derived from the logarithm of the seismographamplitude, the amount of energy released in an earthquake is not a simple function

of the magnitude – each unit on the Richter scale represents a 32-fold increase

in the energy released

A guide to earthquake magnitude

Magnitude less than 4.5

Magnitude 4.5 represents an energy release of about 10 8 kilojoules and is the equivalent of about 10 tonnes of TNT being exploded underground Below about magnitude 4.5, it is extremely rare for an earthquake to cause damage, although

it may be quite widely felt Earthquakes of magnitude 3 and magnitude 2 become increasingly difficult for seismographs to detect unless they are close to the event A shallow earthquake of magnitude 4.5 can generally be felt for 50 to

100 km from the epicentre.

Magnitude 4.5 to 5.5 – local earthquakes

Magnitude 5.5 represents an energy release of around 10 9 kilojoules and is the equivalent of about 1000 tonnes of TNT being exploded underground Earth- quakes of magnitude 5.0 to 5.5 may cause damage if they are shallow and if they cause significant intensity of ground shaking in areas of weaker buildings Earthquakes up to magnitudes of about 5.5 can occur almost anywhere in the world – this is the level of energy release that is possible in normal non-tectonic geological processes such as weathering and land formation An earthquake of magnitude 5.5 may well be felt 100 to 200 km away.

Magnitudes 6.0 to 7.0 – large magnitude events

Magnitude 6 represents an energy release of the order of 10 10 kilojoules and is the equivalent of exploding about 6000 tonnes of TNT underground A magnitude 6.3

is generally taken as being about equivalent to an atomic bomb being exploded underground A magnitude 7.0 represents an energy release of 10 12 kilojoules Large-magnitude earthquakes, of magnitude 6.0 and above, are much larger energy release associated with tectonic processes If they occur close to the surface they may cause intensities at their centre of VIII, IX or even X, causing very heavy damage or destruction if there are towns or villages close to their epicentre Some of these large-magnitude earthquakes, however, are associated with tectonic processes at depth and may be relatively harmless to people on the earth’s surface There are about 200 large-magnitude events somewhere in the world each decade A magnitude 7.0 earthquake at shallow depth may be felt at distances 500 km or more from its epicentre.

Magnitudes 7.0 to 8.9 – great earthquakes

A magnitude 8 earthquake releases around 10 13 kilojoules of energy, equivalent

to more than 400 atomic bombs being exploded underground, or almost as much as a hydrogen bomb The largest earthquake yet recorded, magnitude 8.9, released 10 14 kilojoules of energy Great earthquakes are the massive energy releases caused by long lengths of linear faults rupturing in one break If they occur at shallow depths they cause slightly stronger epicentral intensities than large-magnitude earthquakes but their great destructive potential is due to the very large areas that are affected by strong intensities.

Very sensitive instruments can record earthquakes with magnitudes as low

as −2, the equivalent of a brick being dropped from the table to the ground.The energy released from an earthquake is similar to an explosive charge beingdetonated underground, with magnitude being the measure of the energy released

In the guide to magnitude (see box), an explosive equivalent of each magnitudelevel is given as a rough guide The destructive effects at the earth’s surface of theenergy released are also affected by the depth of the earthquake: energy releasedclose to the surface will be more destructive on the area immediately above it,and a deep energy release will affect a wider area above, but the energy will bemore dissipated and the effects weaker

1.3.10 Limits to Magnitude

The larger the area of fault that ruptures, and the bigger the movement that takesplace in one thrust, the greater the amount of energy released The length ofthe fault and its depth determine the area of its rupture: in practice the depth

of rupture is constrained by the depth of the earth’s solid crust, so the criticalparameter in determining the size of earthquake is the length of the fault rupturethat takes place The tectonic provinces where long, uninterrupted fault lengthsexist are limited, and are by now fairly well defined The limits to magnitudeappear to be the sheer length of fault that could possibly unzip in one singlerupture The largest magnitude earthquake yet recorded measured 8.9, rupturingover 200 continuous kilometres down the coast of Chile

Because of this tendency for magnitude scales to saturate at about 9, ogists have developed a new measure of the magnitude of an earthquake which

seismol-derives more directly from the source characteristics Seismic moment is defined

by the rigidity of the rocks, multiplied by the area of faulting, multiplied by theamount of the slip Seismic moment can be inferred from instrument readings,and for larger earthquakes checked by observations of the surface fault trace

Based on seismic moment, a moment magnitude ( Mw) has been defined whichcorrelates well with other measures of magnitude over a range of magnitudes

1.3.11 Intensity

Intensity is a measure of the felt effects of an earthquake rather than the quake itself It is a measure of how severe the shaking was at any location.For any earthquake, the intensity is strongest close to the epicentre and atten-uates away with distance from the source of the earthquake Larger magnitudeearthquakes produce stronger intensities at their epicentres Intensity mapping

earth-showing isoseismals, or lines of equal intensity, is normally carried out after

each damaging earthquake by the local geological survey Isoseismal maps of